Hello, I usually try to give the least expensive fix for a problem, but in this case, I’ll suggest more expensive actions that fix the design problem as well. The bigger heater should never have been shoe-horned into such a tight space, as you can’t service it now. So, I’d either get a shorter glass-lined tank or a Marathon tank, (has no anode) to replace the existing one. It’s hard to know what’s happened inside of a tank that’s been sitting stagnant for three years. There may be little life left in it as the water has been made more conductive with all the bacteria growing in it. More conductive water speeds anode consumption.
If there is room to service the smaller heater, then putting a good drain valve, curved dip tube and an aluminum/zinc anode in it will likely do the trick… if the water is used consistently. If it sits unused for periods of time, a powered anode or a Marathon tank are good options. 😎